Many operating machines, such as fork-lift trucks, shovels and the like offer the possibility of electronically controlling the transmission of movement from the motor to the wheels.
As technicians in the sector know, these operating machines have a function known as “inching” which allows the operator to reduce and control the speed of the operating machine by means of the brake pedal, thus changing the transmission conditions so as to adjust the traction.
During the first part of the brake pedal stroke, the “inching” function allows reducing the traction without operating the brakes.
More in particular, during the first part of the pedal stroke, the operator modulates the speed of the operating machine by adjusting the movement transmission speed, while in the second part of the stroke, the further movement of the pedal causes disengagement of the transmission and the direct operation of the brakes.
As technicians in the sector know, the “inching” function can be controlled by means of a command apparatus comprising a braking valve or a master cylinder.
In jargon, the master cylinder is also called “pump-brake”, a definition this that will be used in the rest of the text.
The present invention relates to the type that makes use of a pump-brake for the control of the inching function.
As it is known, the control apparatuses belonging to this manufacturing type generally comprise:
a feeding line of an operating fluid connected to a collection tank;
a delivery line connected to the braking system;
a pump-brake which comprises a containment body having a chamber communicating with the feeding line and with the delivery line and a piston sliding in such chamber to distribute the operating fluid towards the delivery line;
control means, such as the brake lever or pedal, suitable for controlling mechanically the movement of the piston;
and at least a control device suitable for detecting the movements of the control means in such a way as to modify the transmission during the first part of their movement.
More in particular, the control devices used envisage the presence of a fixed part and a moving part, where the latter is operatively connected to the control means in such a way as to be affected by their movement.
The known command apparatus which use a pump-brake to control the inching function differ according to the ways the movement of the moving part of the control device is performed.
A first known solution envisages that the moving part of the control device be associated integral with the control means, which are directly connected to the pump-brake piston.
This first solution does however have a number of drawbacks.
In fact, such solution necessarily requires that the stroke of the control means be restricted in order to avoid the pressure inside the pump-brake from rising too far before the transmission has been completely disengaged with a consequent overheating effect that could cause damage to the vehicle.
A second known solution envisages that a part of the control device be associated integral with the control means and that the other part be associated with the pump-brake piston. The connection between the control means and the piston is achieved in such a way that the first part of the stroke of the control means themselves does not involve any movement of the piston but only the relative movement of the control device components. At the end of this first part of the stroke, the movement is interrupted relating to the parts of the control device and the further movement of the control means results in the movement of the piston and therefore the start of the braking stage.
This second solution also has its drawbacks however.
In fact, because the first part of the stroke of the control means does not result in any piston movement, it follows that the second part of the stroke of the control means (which therefore starts after the relative movement between the components of the control device has stopped and after the traction has been deactivated) must cause both the calipers to move closer to the brakes and the braking stage itself.
Because the braking stage takes place during a reduced stroke of the control means, the pump-brake piston must have a large diameter, which consequently results in a heavy load on the control means and therefore considerable effort on the part of the user.
A third solution of known type envisages that the control device be positioned along the delivery line of the pump-brake and that it switches on therefore following the increase in pressure along the delivery line itself.
This solution too has its drawbacks.
In fact, the positioning of the control device along the delivery line of the pump-brake results in the transmission only being reduced after the braking action has begun.